than a hard drive, this is a random-access read/write magnetic media.
■ Spin-Transfer Torque RAM: stores bits
as spin-aligned electrons rather than
magnetically aligned atoms. Right
now, this technology leads in the
shortest write latencies.
■ Phase-Change RAM: stores bits by
changing the crystalline structure of a
Fortunately, that alloy crystal can be
embedded in a small transistor and can
change phase in 10s of nanoseconds.
■ Programmable Metallization RAM:
stores bits by switching the ionization
of atoms in an electrolyte between
■ Resistive RAM (memristors): stores bits
by changing the conductive properties
of a dielectric cell.
These new chips are all nano-fast for
writes and reads without any buffers and
can store data without any active power
supply. But how does that persistence
compare to disk drive reliability? In
fact, storage to NVM chips is almost as
reliable as disk storage now, and it will
become more reliable in ways that disks
are unlikely to achieve. Current disks
offer an endurance of about 1015 read/
write cycles; current SRAM/DRAM can
handle 1016 cycles—without persistence.
Right now, NVM technologies offer about
1014–1015, but should be able to hit
1016–1017 cycles and more. That’s several
decades of storage stability, matching
and exceeding hard drives.
Flash in the Pan
But isn’t Flash SSD a pretty good
answer, available now? Yes, but this
is a temporary technology, bringing
some benefits now, but slated for quick
obsolescence. Flash is better than a disk
drive, but production Flash NAND gates
are slow. Fast writes are possible only
with a large RAM write cache stuck into
the Flash memory cards. Worse, the
gates are good for only 105–106 cycles—
each electronic write damages the cell,
so large Flash devices need additional
circuitry for leveling writes across
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